Conventional electrical strain gauges require a relatively large number of electrical lead outs which increases the weight and expense. Additionally such conventional electrical strain gauges are subject to electromagnetic noise which can give rise to false or distorted readings. This makes such conventional gauges relatively unsatisfactory for use on aircraft either as the retrofit assembly or as part of a composite panel assembly. Moreover in an aircraft application the use of electrical cables means that they can be damaged by lightning strikes with consequent reduction in reliability and life of the strain gauge assembly.